看完ReaderT的源码，接着来分析WriterT的源码。

示例

先看一个超级简单的例子：

import Control.Monad
import Control.Monad.Trans.Writer

data LogEntry = LogEntry { msg :: String } deriving (Eq, Show)

calc :: Writer [LogEntry] Integer
calc = do
    tell [LogEntry "Start"]
    let x = sum [1 .. 10000000]
    tell [LogEntry (show x)]
    tell [LogEntry "done"]
    return x
    
test = execWriter calc
test2 = runWriter calc

test执行结果：

[LogEntry {msg = "Start"},LogEntry {msg = "50000005000000"},LogEntry {msg = "done"}]

ReaderT是Haskell里很常见的一种模式。比如读取环境配置：

mio 是 Metal IO，Rust语言生态比较底层的I/O库，官网的介绍：

Mio is a lightweight I/O library for Rust with a focus on adding as little overhead as possible over the OS abstractions.

mio目前已经发布了v0.6.19版本，这次分析代码版本选择 master分支，commit id 14f37f283576040c8763f45de6c2b2bbcb82436d

我们从官方自带的example进行源码跟踪分析。